U.S. Pat. No. 4,921,767, issued to Datta et al., on May 1, 1990, describes the basic method of manufacturing a luminescent screen for a color CRT by the electrophotographic screening (EPS) method, using dry-powdered, triboelectrically charged screen structure materials that are serially deposited onto a suitable photoreceptor disposed on an interior surface of a faceplate panel. The photoreceptor comprises, preferably, an organic conductive (OC) layer and an overlying organic photoconductive (OPC) layer. In the above-referenced patent, the OPC layer is a volatilizable organic polymeric material such as polyvinyl carbazol, n-ethyl carbazole, n-vinyl carbazole or tetraphenyl butatriene (TPBT) dissolved in a suitable binder, such as polymethyl methacrylate or polypropylene carbonate. A drawback of these OPC materials is that they tend to develop cracks when held for a period of about 48 hours, at a relative humidity of about 75%. Forty eight hours represents a typical weekend during which the manufacturing operation may be shut down. Faceplate panels, having such a OPC layer deposited on a Friday and held in an uncontrolled environment over the weekend, may be unusable the following Monday.
U.S. Pat. No. 5,135,826, issued on Aug. 4, 1992 to Ritt et al., describes a method of manufacturing a screen assembly by the EPS process wherein a suitable quantity of a dialkyl phthalate plasticizer is added to the solution which is used to form the OPC layer. The plasticizer permits the panels, coated with a photoreceptor, to be held over a weekend without any special humidity control. Panels coated with a photoreceptor having the above-mentioned plasticizer in the OPC layer display acceptable electrical characteristics when charged for 30 seconds in an ambient atmosphere of 21.degree. C. and 68% RH. However, panels made using the cabazole-containing materials exhibit a tendency to develop cracks during filming, and a long time is required to bakeout the volatilizable constituents of the OPC layer during screen bake. Panels made using TPBT exhibit a tendency of the OPC layer to crystallize, and the layer has no appreciable sensitivity in the wavelength of current interest, i.e., 400-500 nm. The crystallization is objectionable because electrical breakdown occurs at the crystal sites, and produces phosphor and/or matrix defects at these sites.
An improved OC layer includes certain quarternary ammonium polyelectrolytes recited in U.S. Pat. No. 5,370,952 issued on Dec. 6, 1994 to Datta et al. Preferably the OPC layer is formed from a solution containing polystyrene; an electron donor material, such as 1,4-di(2,4-methyl phenyl)-1,4 diphenylbutatriene (hereinafter 2,4-DMPBT); an electron acceptor material, such as 2,4,7-trinitro-9-fluorenone (herinafter TNF) and 2-ethylanthroquinone (hereinafter 2-EAQ); and a solvent, such as toluene or xylene. The preferred OPC layer possesses fewer of the shortcomings of the prior materials. However, it has been determined that even with the improved OPC layer, certain steps in the EPS process can be further optimized by controlling at least the relative humidity as well as the temperature, during these process steps.